Identification of agents that increase glucose uptake

ABSTRACT

The present invention relates to the use of NAD(P)H oxidase inhibitors to increase cellular uptake of glucose and in the treatment and/or prevention of diseases caused by insulin resistance or diseases related thereto, such as type II diabetes. Specifically, the invention relates to a method for identifying an agent useful for the treatment or prophylaxis of a medical condition associated with elevated levels of blood glucose, the method comprising (i) contacting a candidate agent with a mammalian NAD(P)H oxidase or NAD(P)H oxidase complex; and (ii) determining whether said candidate agent inhibits the biological activities of the NAD(P)H oxidase or NAD(P)H oxidase complex.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Swedish Patent Application No. 0201152-6, filed Apr. 17, 2002, and U.S. Provisional Patent Application Ser. No. 60/410,626, filed Sep. 13, 2002. These applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to the use of NAD(P)H oxidase inhibitors to increase cellular uptake of glucose and in the treatment and/or prevention of diseases caused by insulin resistance or diseases related thereto, such as type II diabetes.

BACKGROUND ART

A large number of people suffer, or are predisposed to suffer from disturbances in their metabolism. One such disturbance includes insulin resistance, which is characteristic of the metabolic syndrome (syndrome X), polycystic ovary syndrome, obesity and type II diabetes, diseases that are rapidly growing in number in the western world. These diseases are multi-factorial and their mechanism or physiology are, in the majority of cases, not well characterized or understood. Type II diabetes includes the most prevalent form of diabetes, which results from insulin resistance with an insulin secretory defect. Pharmacological treatments such as metformin and rosiglitazone have an ameliorating effect on insulin resistance and are believed to increase the effectiveness of endogenous insulin and thereby contribute to the lowering of elevated blood glucose levels in type II diabetes patients.

One mechanism whereby insulin resistance may be induced is via elevation of reactive oxygen species (ROS). Although contrasting effects of ROS have been reported on the insulin signal transduction system and glucose transport, it has been shown that prolonged exposure of cells to ROS causes insulin resistance. Insulinomimetic effects of ROS have been reported using muscle cells and adipocytes. Acute exposure of adipocytes to H₂O₂ was shown to activate pyruvate dehydrogenase activity and lipid synthesis [May et al., Journal of Biological Chemistry, 254:9017-21 (1979)]. Some but not all aspects of insulin signaling appear to be activated by H₂O₂. Using L6 myocytes it was shown that H₂O₂ caused a P13K-dependent activation of PKB and inhibition of GSK3 within 30 min of treatment [Tirosh et al., Journal of Biological Chemistry, 274:10595-602 (1999)]. Prolonged treatment (24 h) of L6 muscle cells and 3T3-L1 adipocytes with a ROS generating system increased the expression of GLUT1 that resulted in elevated basal glucose transport [Kozlovsky et al., Free Radical Biology & Medicine, 23:859-69 (1997); Kozlovsky et al., Journal of Biological Chemistry, 272:33367-72 (1997)]. Treatment of these cell lines with H₂O₂ also interferes with insulin signaling [Rudich et al., American Journal of Physiology, 272:E935-40 (1997)]. Simultaneous treatment with insulin and H₂O₂ was shown to inhibit insulin stimulated glucose transport and glycogen synthesis in spite of intact PKB activation [Blair et al., Journal of Biological Chemistry, 274:36293-9 (1999)]. Pretreatment with ROS inhibited insulin stimulated IRS-1 and P13K cellular redistribution, PKB serine phosphorylation and glucose transport [Tirosh, Potashnik et al., Journal of Biological Chemistry, 274:10595-602 (1999)]. The antioxidant lipoic acid could prevent these effects [Rudich et al., Diabetologia, 42:949-57 (1999)]. Taken together, these results suggest that insulin signaling involve redox reactions, with some steps that can be mimicked and some that can be inhibited by H₂O₂. Integrating these findings with the demonstration that insulin can stimulate the production of H₂O₂, it can be hypothesized that ROS are involved in insulin signaling and may be responsible for the insulin resistance observed after prolonged treatment with insulin and other agents.

Oxidative stress is caused by excess free radical production in cellular metabolism. The free radicals derived from reaction products of oxygen are often termed reactive oxygen species (ROS). A reducing environment inside the cell prevents oxidative damage and can be maintained by the action of antioxidant enzymes and substances, such as superoxide dismutase (SOD), catalase, glutathione, selenium-dependent glutathione, thioredoxin hydroperoxidases, thioredoxin, vitamins C and E, and probably more unknown players.

Oxidative stress has been demonstrated in several different diseases and is implicated as an important driving force in the aging process [Finkel et al., Nature, 408:239-47 (2000); Spector, Journal of Ocular Pharmacology & Therapeutics, 16:193-201 (2000)]. A growing body of data demonstrate signs of increased oxidative stress in type II diabetes. It is likely that the oxidative stress is contributing to many of the vascular complications occurring in the late stages of the disease but the evidence for oxidative stress as causative factor in the development of insulin resistance and deterioration of beta cell function is still lacking. An inverse relationship between insulin action on glucose disposal and plasma superoxide ion, and a positive relationship between insulin action on glucose disposal and plasma GSH/GSSG ratio have been observed in type 2 diabetic patients during euglycemic hyperinsulinemic clamp [Paolisso et al., Metabolism: Clinical & Experimental, 43:1426-9 (1994)]. Decreased serum vitamin E content, a marker of impaired oxidant/antioxidant status, was reported to be associated with increased risk of developing type II diabetes [Salonen et al., BMJ, 311:1124-7 (1995)]. In animal experiments it was recently demonstrated that chemically induced oxidative stress exacerbated insulin resistance and hyperglycemia in obese Zucker rats [Laight et al., British Journal of Pharmacology, 128:269-71 (1999)]. There are also indications that beta cell toxic agents like alloxan and streptozotocin that are used to induce experimental animal diabetes act via oxidative stress [Davis et al., Biochemical Pharmacology, 55:1301-7 (1998); Hotta et al., Journal of Experimental Medicine, 188:1445-51 (1998)].

Superoxide can be produced by a number of cellular enzyme systems: NAD(P)H oxidases, xanthine oxidase, lipoxygenases, cyclooxygenase, P-450 monooxygenases, and the enzymes of mitochondrial oxidative phosphorylation. The majority of free radicals are produced by the mitochondria as unwanted by-products of the respiratory chain but the cell also purposely generates free radicals. The cellular defense system of the body utilizes oxygen radicals to kill invading microorganisms and the vascular system uses the nitric oxide radicals as an intermediate in the regulation of vascular tone. Originally, the NAD(P)H oxidase system responsible for production of superoxide that participates in bacterial killing was demonstrated in neutrophils and other phagocyte cells [Segal et al., Annals of the New York Academy of Sciences, 832:215-22 (1997)]. A growing number of experimental data from endothelial cells and other cell types show that ROS can be produced through activation of NAD(P)H-oxidase [Jones et al., American Journal of Physiology, 271:H1626-34 (1996); Krieger-Brauer et al., Journal of Biological Chemistry, 272:10135-43 (1997); Bayraktutan et al., Cardiovascular Research, 38:256-62 (1998)]. When activated, the NAD(P)H oxidase assembles at the plasma membrane and catalyses the single electron reduction of molecular O₂ to superoxide (O₂ ⁻). In the presence of superoxide dismutase, O₂ ⁻ dismutates to hydrogen peroxide (H₂O₂) that can be converted to a hydroxyl radical (OH⁻) in the presence of ferrous ions. The list of other free radicals originating from O₂ ⁻ that can be formed in the cell is longer, and will not be further discussed here. At least five proteins are required for the formation of an active NAD(P)H oxidase complex: the membrane bound cytochrome b558 and the cytosolic proteins, p47^(phox), p67^(phox), p40^(phox) and a small GTP-binding protein, Rac-1 or Rac-2 [Abo et al., Journal of Biological Chemistry, 267:16767-70 (1992); Babior, Advances in Enzymology & Related Areas of Molecular Biology, 65:49-95 (1992); Knaus et al., Journal of Biological Chemistry, 267:23575-82 (1992)]. Cytochrome b558 is a flavoprotein with an NAD(P)H-binding site and consists of two subunits, gp91^(phox) and p22^(phox) [Sumimoto et al., Biochemical & Biophysical Research Communications, 186:1368-75 (1992)].

The hypoglycemic agent diphenylene iodinium (DPI) has been shown to diminish the rate of mitochondrial respiration by inhibiting NADH dehydrogenase. Holland et al. (1973; J. Biol. Chem. 248: 6050-6056) discloses that the enzyme inhibition causes the hypoglycemic action by decreasing mitochondrial oxidation and the hepatic and whole body ATP content (See also Gatley, S. J. & Martin, J. L. (1979) Xenobiotica 9: 539-546). However, it has not been previously shown that agents which inhibit NAD(P)H oxidase would be useful for increasing the activity of the insulin receptor and/or the intracellular insulin-signaling pathway, and thereby be useful against insulin resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the effect of DPI on insulin stimulated glucose transport in L6 cells when treated with different concentrations (0.1-10 μM) of DPI alone for 30 min or together with insulin (200 or 1000 nM) for additional 30 min.

FIG. 2 is a graph depicting the effect of H₂O₂ generated by glucose oxidase (GO) DPI stimulated glucose transport. Differentiated L6 cells were treated with 25 mU/ml GO for 30 min before addition of DPI. After additional 30 min, 200 nM insulin was added and glucose transport was measured after 30 min.

FIG. 3 is a graph depicting blood glucose concentrations during an insulin tolerance test in ob/ob mice treated for 4 days with daily i.p. injections of DPI, 1 mg/kg (n=7), or vehicle (n=8).

DISCLOSURE OF THE INVENTION

It has surprisingly been found that inhibition of NAD(P)H oxidase stimulates glucose uptake in rat skeletal muscle cells. A NAD(P)H oxidase complex is putatively involved in down-regulation of insulin signaling via generation of ROS. Thus, pharmacological inhibition of NAD(P)H oxidase activity should increase insulin signaling and restore insulin sensitivity. This surprising effect has not been seen previously and demonstrates the utility of the entire, or parts of, NAD(P)H oxidase complex, which generates ROS, as a tool for finding drugs that can be used for treating type II diabetes, specifically insulin resistance.

Consequently, in a first aspect this invention provides a method for identifying an agent useful for the treatment or prophylaxis of a medical condition associated with elevated levels of blood glucose, said method comprising

(i) contacting a candidate agent with a mammalian NAD(P)H oxidase or NAD(P)H oxidase complex; and

(ii) determining whether said candidate agent inhibits the biological activities of the NAD(P)H oxidase or NAD(P)H oxidase complex.

The said medical condition is preferably associated with insulin resistance, such as, in particular, type II diabetes. One clinical definition of diabetes is the so-called fasting glucose level. A patient is diagnosed with diabetes if the amount of glucose is above 126 milligrams per deciliter (mg/dl) measured on two occasions. Impaired fasting glucose and impaired glucose tolerance are associated with the insulin resistance syndrome. An individual can be insulin resistant in the absence of fasting hyperglycemia if an oral glucose tolerance test with 75 g anhydrous glucose dissolved in water gives a 2 h plasma glucose value≧200 mg/dl in a test performed as described by WHO [World Health Organization, Tech. Rep. Ser., no. 727, (1985)].

In one embodiment of the invention, cells containing the NAD(P)H oxidase or the NAD(P)H oxidase complex may be brought into contact with inhibitors of the NAD(P)H oxidase or the NAD(P)H oxidase complex, followed by monitoring the glucose uptake by these cells, and comparing this activity with that of a the NAD(P)H oxidase or the NAD(P)H oxidase complex in the absence of inhibitor. Compounds that affect the glucose uptake of these cells are to be considered as potential drug candidates.

The NAD(P)H oxidase or NAD(P)H oxidase complex is preferably selected from the group consisting of gp91phox, p22phox, Mox2, Nox4, Nox5, DUOX1, DUOX2, (b5+b5R) oxidoreductase, p47phox, p67phox, p4Ophox, Rac-1, and Rac-2.

The proteins may be of any mammalian species, however, a preferred species is Homo sapiens. The nucleotide and amino acid sequences from Homo sapiens are disclosed in the enclosed sequence listing.

In one embodiment, the invention includes a method for identifying an agent that increases glucose uptake by a cell. The method includes the following steps: contacting a cell with a candidate agent that inhibits the activity of an NAD(P)H oxidase or an NAD(P)H oxidase complex; measuring glucose uptake by the cell in the presence of the candidate agent; and determining whether the candidate agent increases glucose uptake by the cell.

The method can optionally include an additional step of comparing glucose uptake by the cell in the presence of the candidate agent with glucose uptake by a cell in the absence of the candidate agent.

The method can optionally include a step of contacting the candidate agent with the NAD(P)H oxidase or the NAD(P)H oxidase complex and determining that the candidate agent inhibits the activity of the NAD(P)H oxidase or the NAD(P)H oxidase complex.

In one example, the NAD(P)H oxidase or the NAD(P)H oxidase complex is a human NAD(P)H oxidase or a human NAD(P)H oxidase complex. The NAD(P)H oxidase or the NAD(P)H oxidase complex can optionally be selected from the group consisting of gp91phox, p22phox, Mox2, Nox4, Nox5, DUOX1, DUOX2, (b5+b5R) oxidoreductase, p47phox, p67phox, p4Ophox, Rac-1, and Rac-2. The nucleic acid and amino acid sequences of exemplary NAD(P)H oxidases and NAD(P)H oxidase complexes are described herein.

The cell can be, for example, a muscle cell or an adipocyte.

In some embodiments, the candidate agent is selected from the group consisting of pyridine, imidazole, diethyl pyrocarbonate, chloromercuribenzoic acid, 4-(2-aminomethyl)-sulfonyl fluoride acetovanillone, and derivatives thereof.

In another aspect, the invention features a method for increasing glucose uptake in a cell by contacting a cell with an amount of an inhibitor of an NAD(P)H oxidase or an NAD(P)H oxidase complex effective to increase glucose uptake by the cell. The cell can be, for example, a muscle cell or an adipocyte. The method can optionally include an additional step of detecting an increase in glucose uptake by the cell in response to the contacting of the cell with the inhibitor.

In one example, the NAD(P)H oxidase or the NAD(P)H oxidase complex is a human NAD(P)H oxidase or a human NAD(P)H oxidase complex. The NAD(P)H oxidase or the NAD(P)H oxidase complex can optionally be selected from the group consisting of gp9lphox, p22phox, Mox2, Nox4, Nox5, DUOX1, DUOX2, (b5+b5R) oxidoreductase, p47phox, p67phox, p4Ophox, Rac-1, and Rac-2. The nucleic acid and amino acid sequences of exemplary NAD(P)H oxidases and NAD(P)H oxidase complexes are described herein.

In some embodiments, the inhibitor is selected from the group consisting of pyridine, imidazole, diethyl pyrocarbonate, chloromercuribenzoic acid, 4-(2-aminomethyl)-sulfonyl fluoride acetovanillone, and derivatives thereof.

In another aspect, the invention provides a method for the treatment of a medical condition associated with elevated levels of blood glucose, comprising administering to a patient in need thereof an effective amount of an inhibitor or antagonist of NAD(P)H oxidase or NAD(P)H oxidase complex.

In one embodiment, the invention features a method for the treatment of a medical condition, including the following steps: selecting an individual diagnosed as having a medical condition characterized by elevated levels of blood glucose; and administering to the individual an amount of an inhibitor of an NAD(P)H oxidase or an NAD(P)H oxidase complex effective to reduce blood glucose levels in the individual.

The medical condition can be characterized by, for example, insulin resistance, a need for increased activity of the insulin receptor, and/or a need for increased activity of the intracellular insulin-signaling pathway.

In one example, the medical condition is diabetes, e.g., type II diabetes.

In some embodiments, the individual does not have and/or has not been diagnosed as having a disorder (e.g., atherosclerosis) characterized by a vascular injury, e.g., vascular hyperpermeability of endothelial cells. In addition, in some embodiments, the method does not include a step of evaluating a vascular injury (if present) in the individual before and/or after the administration of the inhibitor to the individual.

In some embodiments, the method includes an additional step of detecting a reduction in blood glucose levels in the individual in response to the administration of the inhibitor.

In one example, the NAD(P)H oxidase or the NAD(P)H oxidase complex is a human NAD(P)H oxidase or a human NAD(P)H oxidase complex. The NAD(P)H oxidase or the NAD(P)H oxidase complex can optionally be selected from the group consisting of gp91phox, p22phox, Mox2, Nox4, Nox5, DUOX1, DUOX2, (b5+b5R) oxidoreductase, p47phox, p67phox, p4Ophox, Rac-1, and Rac-2. The nucleic acid and amino acid sequences of exemplary NAD(P)H oxidases and NAD(P)H oxidase complexes are described herein.

In some embodiments, the inhibitor is selected from the group consisting of pyridine, imidazole, diethyl pyrocarbonate, chloromercuribenzoic acid, 4-(2-aminomethyl)-sulfonyl fluoride acetovanillone, and derivatives thereof.

The inhibitor or antagonist can be identified according to the method as described above. Examples of known inhibitors or antagonists are those selected from the group consisting of pyridine, imidazole, diethyl pyrocarbonate, chloromercuribenzoic acid, 4-(2-aminomethyl)-sulfonyl fluoride and acetovanillone, including derivatives thereof. The said inhibitor or antagonist is e.g. a compound having an inhibitory effect on the ROS generating activity of the NAD(P)H oxidase or the NAD(P)H oxidase complex. The inhibitor or antagonist could exert its effect by interacting with the active site or a regulatory site, or both sites, of the NAD(P)H oxidase.

A compound that shows the desirable characteristics with regards to inhibiting the activity of the NAD(P)H oxidase or the NAD(P)H oxidase complex will be further tested in an assay of insulin stimulated glucose uptake in differentiated L6-K1 cells or other skeletal muscle cells, muscle tissue biopsies, adipocytes or adipocyte cell lines. An active compound should stimulate basal and insulin stimulated glucose uptake in a manner similar to the NAD(P)H oxidase inhibitor diphenylene iodonium (DPI). The compounds will preferably be of such nature that they are suited for oral administration, but any route of administration, such as, intravenous, suppository or parental routes will be considered.

In yet another aspect, the invention provides the use of an NAD(P)H oxidase- or NAD(P)H oxidase complex inhibitor or antagonist, as described above, in the manufacture of a medicament for the treatment and/or prevention of a medical condition connected with elevated levels of blood glucose.

As defined herein, the term “prevent” or “treat” is not intended to exclusively mean the complete abolishment of the disease or condition, but is meant that there is complete or some amelioration, so that an improvement over the expected symptomology is clinically observed. An example of one such criterion could be the lowering of blood glucose levels by more than 25%. Other such criteria, well known in the art, could be envisioned.

As defined herein, the term “reactive oxygen species” means compounds selected from the group comprising compounds or compound species such as H₂O₂, OH⁻ and O₂ ⁻. Compounds such as these will be referred to as “ROS”.

As defined herein, the term “NAD(P)H oxidase” or “NAD(P)H oxidase complex” means one of the proteins or any combination of two or more of the proteins selected from the group comprising the membrane bound cytochrome b558 consisting of gp₉₁ ^(phox) (nucleotide sequence according to SEQ ID NO:1, amino acid sequence according to SEQ ID NO:2), p₂₂ ^(phox) (nucleotide sequence according to SEQ ID NO:3, amino acid sequence according to SEQ ID NO:4), Mox2 (nucleotide sequence according to SEQ ID NO:5, amino acid sequence according to SEQ ID NO:6), Nox4 (nucleotide sequence according to SEQ ID NO:7, amino acid sequence according to SEQ ID NO:8), Nox5 (nucleotide sequence according to SEQ ID NO:9, amino acid sequence according to SEQ ID NO:10), DUOX1 (nucleotide sequence according to SEQ ID NO: 11, amino acid sequence according to SEQ ID NO:12), p138Tox (DUOX2) (nucleotide sequence according to SEQ ID NO: 13, amino acid sequence according to SEQ ID NO: 14), (b5+b5R) oxidoreductase (nucleotide sequence according to SEQ ID NO: 15, amino acid sequence according to SEQ ID NO: 16), and the cytosolic proteins, p₄₇ ^(phox) (nucleotide sequence according to SEQ ID NO: 17, amino acid sequence according to SEQ ID NO: 18), p67^(phox) (nucleotide sequence according to SEQ ID NO: 19, amino acid sequence according to SEQ ID NO:20), p40^(phox) (nucleotide sequence according to SEQ ID NO:21, amino acid sequence according to SEQ ID NO:22),and a small GTP-binding protein, Rac-1, (which has two different amino acid variants), (nucleotide sequence according to SEQ ID NO:23, amino acid sequence according to SEQ ID NO:24 and SEQ ID NO:25, respectively), or Rac-2, (nucleotide sequence according to SEQ ID NO:26, amino acid sequence according to SEQ ID NO:27), which combination gives rise to reactive oxygen species, or other proteins or assemblies of proteins which essentially have NAD(P)H oxidase activity. Preferably, these enzymes contain consensus sequences for FAD- and/or NAD(P)H-binding sites.

In addition to the specific NAD(P)H oxidase amino acid and nucleotide sequences described herein, fragments or variants thereof that retain NAD(P)H oxidase activity (or fragments or variants thereof that encode polypeptides that retain such activity) can be used in the methods of the invention (e.g., screening methods).

In some embodiments, a polypeptide used in a method of the invention differs from an NAD(P)H oxidase amino acid sequence described herein at one or more residues and yet retains NAD(P)H oxidase activity. The differences are, preferably, differences or changes at a non-essential residue or a conservative substitution. In one embodiment, a polypeptide includes an amino acid sequence that is at least about 60% identical to an NAD(P)H oxidase amino acid sequence described herein or a fragment thereof. Preferably, the polypeptide is at least 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more identical to an NAD(P)H oxidase amino acid sequence described herein. Preferred polypeptide fragments are at least 10%, preferably at least 20%, 30%, 40%, 50%, 60%, 70%, or more, of the length of an NAD(P)H oxidase amino acid sequence described herein.

As used herein, “% identity” of two amino acid sequences, or of two nucleic acid sequences, is determined using the algorithm of Karlin and Altschul (PNAS USA 87:2264-2268, 1990), modified as in Karlin and Altschul, PNAS USA 90:5873-5877, 1993). Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. (J. Mol. Biol. 215:403-410, 1990). BLAST nucleotide searches are performed with the NBLAST program, score=100, wordlength=12. BLAST protein searches are performed with the XBLAST program, score=50, wordlength=3. To obtain gapped alignment for comparison purposes GappedBLAST is utilized as described in Altschul et al (Nucleic Acids Res. 25:3389-3402, 1997). When utilizing BLAST and GappedBLAST programs the default parameters of the respective programs (e.g., XBLAST and NBLAST) are used to obtain nucleotide sequences homologous to a nucleic acid molecule described herein.

The term “NAD(P)H oxidase activity”, as described herein, refers to enzymatic activity of either the NAD(P)H oxidase or the NAD(P)H oxidase complex, as defined herein, whereby reactive oxygen species (ROS) are produced. Such enzymatic activity is readily established and procedures for this are well known to a skilled person. This activity is well known in the art and methods whereby this can be monitored are well known.

The term “inhibiting” with regards to ROS generating activity of the NAD(P)H oxidase or the NAD(P)H oxidase complex is meant the lowering of said activity to the range between 20%-100% of normal activity when measured with said methods. A preferred value of the inhibitory constant K_(i) is <10 μM, or more preferably <1 μM.

As defined herein, the term “NAD(P)H oxidase inhibitor” means any compound capable of lowering the activity of the NAD(P)H oxidase or the NAD(P)H oxidase complex, according to the above mentioned definition.

When activated, the NAD(P)H oxidase complex assembles at the plasma membrane and catalyses the single electron reduction of molecular O₂ to superoxide (°O₂ ⁻). In the presence of superoxide dismutase, O₂ ⁻ dismutates to hydrogen peroxide (H₂O₂) that can be converted to a hydroxyl radical (OH⁻) in the presence of ferrous ions. The list of other free radicals originating from O₂ ⁻ that can be formed in the cell is longer, and will not be further discussed here. Several proteins are required for the formation of an active NAD(P)H oxidase complex and may include: p22phox, Mox2, Nox4, Nox5, DUOX1, DUOX2, (b5+b5R) oxidoreductase, p47phox, p67phox, p40phox, Rac-1, Rac-2 [Lambeth et al. (2000) Trends Biochem. Sci. 25: 459-461].

A fully active complex producing oxygen radicals in the presence of NAD(P)H, FAD, GTP and amphiphilic compounds can be reconstituted in vitro with individual recombinant proteins [Rotrosen et al., Journal of Biological Chemistry, 268:14256-60 (1993)].

The invention will now be demonstrated by the following examples. These examples are for the purpose of illustration only and are not intended to limit the scope of the invention in any way. The information necessary for carrying out these experiments is supplied in the references. Any variations and adjustments that need to be made for correct function of these assays (variations in pH, concentration ranges, etc) will be apparent for a skilled person.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Suitable methods and materials are described below, although methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

EXAMPLES Example 1 The NAD(P)H Oxidase Inhibitor DPI Increases Glucose Uptake in Rat Skeletal Muscle Cells

Cell culture medium, fetal bovine serum, antibiotics, trypsin-EDTA were purchased from Life Technologies. Diphenylene iodonium (DPI), cytidine, bovine insulin, bovine serum albumin, and glucose oxidase were purchased from Sigma. 2-Deoxy-[³H] glucose (specific activity 1102.6 GBq/mmol) was purchased from NEN Life Science Products and 2-Deoxy-[¹⁴C] glucose from Amersham Pharmacia Biotech. Tissue culture plastics were purchased from Becton Dickinson.

Rat skeletal muscle L6-K cells were grown in minimal essential medium (α-MEM Glutamax I) containing 10% fetal bovine serum at 37° C., 5% CO₂. The cells were passaged twice weekly by treatment with trypsin-EDTA and transfer of ⅓ of the cells to new flasks with fresh culture medium. For differentiation into myotubes, 30,000 cells were seeded in 1 ml in 24-well plates. When the cells were confluent, usually after 3 days, the medium was replaced by differentiation medium consisting of α-MEM, 2% fetal bovine serum and penicillin/streptomycin at a concentration of 100 U/ml and 100 μg/ml, respectively. The medium was replaced every 2-3 days. Between days 4-7, differentiation medium containing 1 mM cytidine was used. The cells were differentiated for 8-10 days before being used in experiments.

On the day before the glucose transport assay, the wells of the culture plate were emptied and 1 ml serum free DMEM containing 5 mM glucose and penicillin/streptomycin was added. In some experiments the cells were treated over night with test compounds in 1 ml and additional treatments were added the next day to give a total volume of 2 ml. In these experiments, insulin (100-1000 nM) was added in 0.2 ml. When all treatments were performed after 20-24 h in serum free medium, a total volume of 1 ml was used. The wells were emptied and 0.5 ml prewarmed PBS without Ca²⁺/Mg²⁺ containing 1 μCi/ml radioactive 2-deoxy-glucose added. After 10 min at 37° C., the wells were emptied and washed three times with cold PBS. The cell monolayer was solubilized in 0.5 ml 0.5 M NaOH for 3 h at room temperature. 400 μl was mixed with 8 ml scintillation fluid (Optiphase, Wallac) and counted in a scintillation counter (Packard TriCarb). Two 10-μl aliquots were used for determination of protein concentration using the method according to Bradford (Anal. Biochem., 1976, 72:248-54) from BioRad.

When differentiated L6 cells are incubated with the NAD(P)H oxidase inhibitor DPI [O'Donnell V.B. et al. (1993) Biochem. J. 290: 41-49] a significant increase in glucose uptake can be observed (FIG. 1). This increase is comparable to or greater than that caused by insulin. This effect is seen when cells are stimulated with 0.1-10 μM DPI for 1 h. A bell-shaped dose response curve for DPI with an optimum at 1 μM is recorded. The effect of suboptimal concentrations of DPI during a 1 h treatment could be stimulated further if insulin is added 30 min after DPI. However, insulin has little additional effect when the maximum effect of DPI is reached in the 1 h protocol (FIG. 1). The effective concentrations at which DPI stimulates glucose transport corresponds well to the concentrations inhibiting NAD(P)H oxidase activity in cell free systems [O'Donnell et al., Biochemical Journal, 290:41-9 (1993)]. These results suggest that DPI stimulates glucose transport via activation of the same mechanism as insulin. On the basis of the above results it is postulated that DPI enhances a constitutive activity of the insulin receptor and/or the intracellular insulin-signaling pathway. The existence of such a constitutive activity is suggested from experiments in which adipocytes have been transfected with the tyrosine phosphatase PTP 1B [Chen et al., Journal of Biological Chemistry, 272:8026-31 (1997)]. These data are compatible with DPI augmenting constitutive intracellular signaling via the same pathway that is used by insulin.

Example 2 Glucose Oxidase Reduces the Effect of DPI on Glucose Uptake

Assuming that the enhancing effect of DPI on insulin signaling was due to inhibition of ROS production, it was investigated whether an exogenous source of H₂O₂ could counteract the effect of DPI. To this end, L6 cells were treated with 25 mU/ml of glucose oxidase for 30 min before addition of DPI. Such a treatment has previously been shown to result in a steady production of micromolar concentrations of H₂O₂ that can freely pass the cell membrane and cause inhibition of insulin signaling [Tirosh, Potashnik et al., Journal of Biological Chemistry, 274:10595-602 (1999)].

It was found that glucose oxidase reduced the stimulatory effect of DPI by 68% and insulin stimulated glucose transport by 65% (FIG. 2). The available results suggest that H₂O₂ can counteract the effect of DPI in addition to inducing insulin resistance. This further strengthens the similarity between the effects of insulin and DPI and shows that DPI acts by inhibiting H₂O₂ production. In spite of superoxide being the primary product of NAD(P)H oxidase, H₂O₂ is the main effector in the cell since superoxide is converted to H₂O₂ by superoxide dismutase.

Example 3 DPI Decreases Blood Glucose Levels in Ob/Ob Mice

Studies were conducted in vivo, in an animal model of obesity characterized by insulin resistance. Eight-month old C57BL/6J ob/ob mice (M&B A/S, Denmark) were matched for sex, weight and fasting blood glucose concentrations. The animals were injected intraperitoneally once daily with DPI (1 mg/kg) or water for 4 days. On day 5, the animals were fasted for 2.5 h and then given an i.p. injection of human insulin 0.5 U/kg (Actrapid, Novo Nordisk, Denmark) and their blood glucose levels were monitored for 4 h by sampling from the tail. The glucose concentration was determined using a Glucometer Accutrend Sensor (Roche).

Without any overt side effects of the DPI treatment, the treated animals exhibited significantly lower blood glucose levels than the control group 1-4 h after injection of insulin, suggesting a decreased insulin resistance (FIG. 3).

Example 4 Identification of Agents Inhibiting NAD(P)H Oxidase

Methods to be used for identifying compounds that inhibit the activity of the NAD(P)H oxidase complex are illustrated.

(A) Neutrophil membrane and cytosol assay for superoxide mediated cytochrome c reduction (Diatchuk, V. et al. (1997) J. Biol. Chem. 272: 13292-13301). Sources of neutrophil membranes and cytosol from buffy coats of normal donors are obtainable from the Blood Bank. Enzyme cofactors and cytochrome c for detection of superoxide-mediated reduction are commercially available. The assay is based on a color change that occurs upon reduction of cytochrome c. This change can be measured as a change in light absorbance using a standard microplate spectrophotometer.

(B) Neutrophil membrane+recombinant p₄₇ ^(phox), p67^(phox) and rac1 for superoxide mediated cytochrome c reduction (absorbance) (Nisimoto, Y. et al. (1997) J. Biol. Chem. 272: 18834-18841).

(C) Fully recombinant NAD(P)H oxidase assay for superoxide mediated cytochrome c reduction [Rotrosen, D. et al, (1993) J. Biol. Chem. 268: 14256-14260].

(D) Fluorescence assay, which measures the interaction between rac and p67^(phox) [Nisimoto, Y. et al. (1997) J. Biol. Chem. 272: 18834-18841]. This assay would limit the screening to detection of compounds interfering with this particular step in the activation of the oxidase. The fluorescent GTP analog 2′-(or-3′)-O-(N-methylanthraniloyl)-βγ-imidoguanosine 5′-triphosphate (MANT-GMPPNP, available from Molecular Probes), binds tightly to Rac, and shows an increase in fluorescence when p67^(phox) is added, indicating complex formation. Rac1 and Rac2 bind to p67^(phox) with a 1:1 stoichiometry and with Kd values of 120 nM and 60 nM, respectively.

(E) Binding assay utilizing ¹²⁵I- or fluorescence labeled mastoparan. Mastoparan is a peptide present in wasp venom that has been shown to inhibit NAD(P)H oxidase activation, most likely via its ability to bind to p67^(phox) [Tisch-Idelson, D., et al.(2001) Biochemical Pharmacology 61: 1063-1071].

(F) Test compounds can be analyzed in a nitroblue tetrazolium reduction assay utilizing a thioredoxin-gp91^(phox) fusion protein. This protein has weak diaphorase activity in the presence of NAD(P)H and FAD and is inhibited by DPI.

(G) Test compounds can be added in appropriate amounts to cultured cells. The reactive oxygen species released from said cells may be measured with the use of a probe, resorufin, which becomes fluorescent in the presence of hydrogen peroxide and a peroxidase [Zhou, M. et al., (1997) Anal. Biochem., 253: 162-168].

Intracellular production of ROS can be measured with the use of various cell permeable analogs of dichlorofluorescin acetate as described by Xie, J. I. et al.[(1999) J. Biol. Chem. 274: 19323-19328].

OTHER EMBODIMENTS

It is to be understood that, while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention. Other aspects, advantages, and modifications of the invention are within the scope of the claims set forth below. 

1. A method for identifying an agent that increases glucose uptake by a cell, the method comprising: contacting a cell with a candidate agent that inhibits the activity of an NAD(P)H oxidase or an NAD(P)H oxidase complex; measuring glucose uptake by the cell in the presence of the candidate agent; and determining whether the candidate agent increases glucose uptake by the cell.
 2. The method of claim 1, wherein the NAD(P)H oxidase or the NAD(P)H oxidase complex is a human NAD(P)H oxidase or a human NAD(P)H oxidase complex.
 3. The method of claim 1, wherein the cell is a muscle cell.
 4. The method of claim 1, wherein the cell is an adipocyte.
 5. The method of claim 1, wherein the NAD(P)H oxidase or the NAD(P)H oxidase complex is selected from the group consisting of gp91phox, p22phox, Mox2, Nox4, Nox5, DUOX1, DUOX2, (b5+b5R) oxidoreductase, p47phox, p67phox, p40phox, Rac-1, and Rac-2.
 6. The method of claim 1, wherein the candidate agent is selected from the group consisting of pyridine, imidazole, diethyl pyrocarbonate, chloromercuribenzoic acid, 4-(2-aminomethyl)-sulfonyl fluoride acetovanillone, and derivatives thereof.
 7. The method of claim 1, further comprising comparing glucose uptake by the cell in the presence of the candidate agent with glucose uptake by a cell in the absence of the candidate agent.
 8. The method of claim 1, further comprising contacting the candidate agent with the NAD(P)H oxidase or the NAD(P)H oxidase complex and determining that the candidate agent inhibits the activity of the NAD(P)H oxidase or the NAD(P)H oxidase complex. 